I notice that most pros hit their forehands with both of their feet in the air. Now how do they transfer their weight forward with both of their feet in the air? Or they don't transfer their weight at all and get all their power from hip rotation?

Since no-one is answering i'll give it a go. The feet off the ground isn't aspired for but actually a result of what has gone on beforehand. After loading the right side correctly they uncoil into the shot with the consequence that the feet often leave the ground. For mine it is usually a combination of upward and foreward weight shift out of the loaded position.

John is right, most of the time, the pros aren't "trying" to jump, they are generating a lot of force with their bodies with an "upward" swing and this results in them coming off the ground sometimes.

As far as weight transfer goes, their weight is transferred during the stroke but the actual transfer of weight in a swing tends to follow the path of the racquet. In fact in good players (Agassi for example) his bodyweight is transfered in the exact same angle/direction as that particular swing. Now most pros are using quite an upward angled swing for serious topspin...so much of their weight/force is directed up...not out....and by the time they actually come off the ground the force has already been transferred through the body.

The other exception would be when an advanced player jumps to get his racquet to the ball the way he likes. The jump could be forward, backward, sideways...anything...but this is only because he/she was not in the proper postion and now must compensate by contorting from normal form...again...they are not doing this on purpose...only trying to get the racquet to the ball the way it needs to and letting their body do what it must to ensure it...

It is quite unproductive to think of jumping when hitting...many people jump in an effort to produce force...besides taking attention away from important things...it often results in them coming off the ground too soon. Before force has been properly transferred up through the body to the small body parts....if you develop a sound topspin swing and start swinging very fast and forcefully then you will find that your body will tend to transfer upwards unless you consciously prevent it!